1. Field of Invention
The present invention relates to an improved beneficiation process to produce low ash clean coal from high ash coals. The invention further relates to a system for implementing an improved beneficiation process to produce low ash clean/coal from high ash coals.
2. Description of Related Art
Coal and coal products continue to play an increasingly important role in fulfilling the energy needs of our society. However, it is generally known that most of the raw coals are associated with mineral matter, which makes it unsuitable for efficient utilization, such as carbonization, gasification, combustion, or liquefaction. Hence, demineralization of the raw coal has tremendous application in the metallurgical industries, thermal power plants and other industries, which require coals with low ash content. Accordingly, physical as well as chemical coal cleaning (beneficiation) processes have been explored. In general, physical coal cleaning processes involve pulverizing the coal to remove the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. Although, the cost of preparing the coal exponentially increases with the amount of fine to be treated, there however exists an economic optimum in size reduction. Although, a step of grinding coal even to extremely fine sizes may not be effective in removing all the impurities. Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are generally divided into four categories for example, gravity, flotation, magnetic and electrical.
As compared to the techniques of physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. As far as Indian scenario is concerned, most of the coals present in India are of drift origin and low grade having high mineral matter contents varying from 5% to greater than 50%. Moreover, the mineral matter is finely disseminated inside the coal matrix and is at times firmly bound. Again, since washability characteristics of Indian coal is not good, it is difficult to remove the mineral matter from coal by conventional physical coal washing techniques based on specific gravity difference such as heavy media and dense media etc. Hence, physical methods of beneficiation such as heavy media and froth flotation, are of limited use for applications in coal beneficiation to produce low ash coals for metallurgical coke making and power generation.
Chemical leaching of coal is a fairly-known technology to produce ultra clean where the ash content of the clean is as low as ˜1.0% based on mineralogical composition of the feed coal. There are potential use of this ultra clean coal both as a fuel and nonfuel.
A prior art process reveals that German coal processing plants treated coal with aqueous sodium hydroxide at elevated temperatures and pressures, and thereafter the coal was extracted with aqueous hydrochloric acid. The said process reduced the sulphur and ash content of the coal. (Crawford, BIOS Final Report No. 522, Item No. 30, Feb. 19, 1946, British Intelligence Objectives Sub-committee, London (A.T.I.—118668, Central Documents Office Wright-Patterson Airforce Base, Dayton, Ohio). Subsequently, the U.S. Bureau of Mines evaluated a similar process for treating coal, involving leaching with aqueous sodium hydroxide at 225° C., both with and without a final stage extraction with aqueous hydrochloric acid. In a report made by Reggel et al in 1972, it was concluded that the sequence of sodium hydroxide treatment and hydrochloric acid extraction removed most of the mineral matter originally present in the coal. Am. Chem. Soc. Div. of Fuel Chem. Preprints, 17(1): 44-48. Battelle Memorial Institute had developed a similar process, which is described in Stambaugh et al U.S. Pat No. 4,055,400 of 1977. According, to the said disclosure, an aqueous alkaline slurry of coal is heated at an elevated temperature and pressure to leach out sulphur and mineral matter. The Battelle process may optionally include last stage extraction with aqueous acid to reduce the final ash content. (Stambaugh et al, Hydrocarbon Processing, 54 (7): 115-116 (1975)). Another known process has undergone extensive development at Iowa State University, Ames, Iowa.
The “Ames” process uses oxidative desulphurization in aqueous slurry of sodium carbonate. Typical conditions are 0.2M Na2CO3 at an oxygen partial pressure of about 4 atmosphere and temperature of 120-140 deg. C. for 1-2 hrs. This development was reviewed in detail by Dr. T. D. Wheelock in 1981. (Chem. Eng. Commun., 12;137-159). In one representative test, using temperature of 120-140 deg. C., the total sulfur content of the coal was reduced by 70% and the pyretic sulphur content was reduced by 78%. (Wheelock (1981).
The Japanese patent publication 466/1942 describes a process for removing ash from coal or coke. The Japanese patent publication 23711/1971 discloses a process for removing sulfur and ash from coals. The Japanese patent publication 133487/1980 describes a coal deashing process. The processes of the first and second of the above-said Japanese publications, use an acid or alkali with the application of pressure and heat to dissolve the metallic components for the removal of ash. When practiced under moderate conditions, these processes fail almost to achieve any ash removing effect and are therefore not suitable as the deashing processes. The process disclosed in the third Japanese publication, the oxidation is followed by an acid or alkali treatment akin to the first and second processes and is such that the FeS2 components which are difficult to dissolve are first oxidized and thereafter dissolved. A further prior art provides use of hydrofluoric acid or hydrogen fluoride gas for treatment of coal since SiO2 is not easily soluble in acids or alkalis to separate Si in the form of gaseous SiF4 to achieve a deashing effect. However, the use of hydrofluoric acid or hydrogen fluoride gas, which is highly toxic and corrosive, involves many difficulties. Thus an actually effective and useful process for removing ash from coal still remains to be developed although the deashing of coal is admittedly a very important technique for the effective use of coal.
The prior art also describes several chemical coal beneficiation processes, for example, U.S. Pat. No. 4,424,062 discloses a process for chemically removing ash coal by immersing ash containing coal in an aqueous solution containing hydrochloric acid or citric acid in combination with acidic ammonium fluoride. U.S. Pat. No. 3,993,455 discloses a process for removing mineral matter from coal by treatment of the coal with aqueous alkali such as sodium hydroxide, followed by acidification with strong acid. Similarly, U.S. Pat. No. 4,55,400 discloses a method of extracting sulphur and ash from coal by mixing the coal with an aqueous alkaline solution, such as ammonium carbonate.
U.S. Pat. No. 4,071,328 discloses a method of removing sulphur from coal by first hydrogenating the coal and the hydrogenated coal was subsequently contacted with an aqueous inorganic acid solution. U.S. Pat. No. 4,127,390 discloses a process for reducing the sulphur content of coal by treatment with an aqueous sodium chloride solution. U.S. Pat. No. 4,134,737 discloses a process for the production of beneficiated coal wherein the coal is digested in caustic, then treated in mineral acid and then treated in nitric acid.
U.S. Pat. No. 4,083,940 discloses a process for cleaning coal by contacting the coal with an aqueous leaching solution containing nitric and hydrofluoric acid. U.S. Pat. No. 4,169,710 discloses comminuting and cleaning coal of sulphur and ash by contacting the coal with a hydrogen halide, such as HF (aqueous and/or anhydrous).
U.S. Pat. No. 4,408,999 discloses beneficiating coal by subjecting the coal to electromagnetic radiation in the presence of a strong inorganic acid, such as hydrofluoric acid. U.S. Pat. No. 4,305,726 discloses a chemical method of removing ash and sulphur from coal, the method comprising treating the coal with hydrochloric and hypochlorous acid in the presence of ferric and ferrous sulphate. U.S. Pat. No. 4,328,002 dislcoses a method of treating coal to remove ash and sulphur involving preconditioning coal particles in the presence of an aqueous solution of an oxidant, such as H2O2 or HF, washing the so-treated coal, treating the washed coal with further oxidant and then passivating the coal with for example, an ammonium salt and then neutralizing with alkali metal hydroxide.
U.S. Pat. No. 4,516,980 discloses a process for producing low-ash, low sulphur coal by a two-stage alkaline treatment using sodium carbonate or bicarbonate as the reagent. The alkaline treated coal is then extracted with aqueous mineral acid; and U.S. Pat. No. 3,998,604 discloses a coal demineralization process whereby ground coal is treated with aqueous acid, such as HCl, H2SO4 or H2CO3 and then subjected to froth flotation in the presence of a gas selected from Cl2so2 or CO2.
All prior art described hereinabove, mostly entail high cost interalia high consumption of energy. Further, the prior art normally deals with coal containing moderately high ash-content coal but not like Indian coals which contain upto 50% of high mineral matter.